The present disclosure relates to an image forming apparatus that forms an image on a sheet, and more particularly to an image forming apparatus having static removers, each of which removes charges from the circumferential surface of a photosensitive drum before a transfer is carried out.
In a known technology, a toner image formed on a photosensitive drum is transferred to a transfer belt as a primary transfer. Thereafter the toner image is transferred from the transfer belt to a sheet as a secondary transfer.
A transfer roller is brought into contact with the photosensitive drum with a transfer belt interposed therebetween. A transfer voltage is applied to the transfer roller under constant current control so that a stable transfer electric field is formed when the primary transfer is carried out.
When the above constant current control is carried out, in a transfer nip part, the difference in electric potential between the non-image part on the photosensitive drum and the transfer roller is likely to be greater than the difference in electric potential between the image part on the photosensitive drum and the transfer roller. When the difference in electric potential between the non-image part on the photosensitive drum and the transfer roller is greater than the difference in electric potential between the image part on the photosensitive drum and the transfer roller, a large amount of transfer current flows into the non-image part on the photosensitive drum. When the transfer current is increased under constant current control to secure transfer performance, current that flows into the non-image part is also increased. This has led to a problem that there is a change in the charging property between the image part and non-image part on the photosensitive drum and a so-called transfer memory occurs.
Technologies described below are known to resolve this transfer memory.
In a technology, charges are removed from the surface of the photosensitive drum before the primary transfer to reduce the difference in electric potential between the image part and the non-image part. When charges are removed from the surface of the photosensitive drum before the primary transfer, however, toner on the image part is likely to scatter to the non-image part.
In another technology, to suppress the above scatter of toner, the electric potential at the non-image part around the image part is made greater than at the image part. Although, with this technology, the difference in potential between the image part and non-image part on the photosensitive drum is comparatively low, a current flow into the non-image part cannot be prevented, so it is difficult to resolve a transfer memory.
In another known technology, charges are removed only from the photosensitive drum in black located at the downstream end in a tandem-type image forming apparatus without dropping the electric potential at the non-image part to prevent the toner in black from scattering. However, this technology causes a transfer memory due to a difference between a transfer current flowing into the image part on the photosensitive drum and a transfer current flowing into the non-image part thereon. Another problem with the tandem-type image forming apparatus is that when toners in a plurality of colors are transferred while being overlapped, toners are likely to noticeably scatter.